A two-faced cant is produced by removing from a log, either by sawing or chipping, an approximately equal volume from each side, after such log has been rotated in an angular position which places its natural curvature in a plane parallel to the two faces to be obtained in the process. Such cants are subsequently resawn perpendicularly to the cut faces to produce lumber of such dimensions as required in the marketplace.
While dimensional accuracy and edge straightness are requisites for this product, it has long been known that some bowing in a board across its thickness dimension, such as may occur in natural fiber stress relieving during the cut, is not detrimental to the quality of merchantable lumber since subsequent stacking and drying processes will straighten out any such curvature to a commercially acceptable degree.
In time, lumber producers have realized that straight line sawing along the length of a naturally curved two-faced cant could entail important losses in lumber yield. This led to a search for means to execute parallel curved cuts in a bowed workpiece, while preserving the thickness accuracy of every board produced. As long as cutting speeds were kept at low levels, simple ways including manual guiding were devised and used with some degree of success. However, during the past 20 years, sawing technology and market pressures have pushed feed rates beyond eight feet per second and new guiding methods for curved sawing had to be found.
Up to present times, most of the improved guiding systems which have been devised for this purpose still involve contact with the rough sides of the two-faced cants being processed. Even if such devices respond to the general curvature of the workpiece, surface deformities alter their perception of the basic shape of the piece. Besides, effective curve limiting in the cut is not possible because the guiding effect depends totally on contact with the natural surfaces of the piece, thus producing at times lumber that is bowed to such an extend as to cause problems in subsequent handling operations.
On the other hand, it is well known that, in making longitudinal cuts perpendicularly to the parallel planar faces of a two-faced cant, any angular misalignment of the feed rolls in contact with said planar faces will cause a deviation of the cut from a straight line. Angular misalignment in this context is considered as any departure from 90° in the angle between the axis of said feed rolls and the theoretical feed line, as viewed in the plane parallel to the planar faces of the cant. A previous development, such as described in the applicant's U.S. Pat. No. 5,400,842 Issued Mar. 28, 1995 to Brisson, has made use of this particularity basically by means of a pivotally mounted feed roll system which has variable orientation in the horizontal plane, thus exerting on the planar faces of the cant a lateral frictional force to cause a deviation of the workpiece during the cut. Since the intent of this development was to use a single guiding unit whether in the front or the back of the saws, it follows that a certain length of the cut, at one end or the other of the piece, is beyond curve control and remains straight. Also, the guiding action caused by the angular misalignment of feed rolls as described above, is dependent on friction factors which vary with the condition of the wood; in any situation, the actual deviation rate of the workpiece tends to be slow in relation to the total process cycling time of one to two seconds, depending on piece length and feed rate. This condition limits to a large extent reversed curve sawing in compound curvature cants.